1. Field of the Invention
The present invention relates in general to optical switching devices. More particularly, the present invention relates to a liquid crystal cross-connect for an optical waveguide and to optical prisms.
2. Technical Background
One of the current trends in telecommunications is the use of optical fibers in place of the more conventional transmission media. One advantage of optical fibers is their larger available bandwidth handling ability that provides the capability to convey larger quantities of information for a substantial number of subscribers via a media of considerably smaller size. Further, because lightwaves are shorter than microwaves, for example, a considerable reduction in component size is possible. As a result, a reduction in material, manufacturing, and packaging costs is achieved. Moreover, optical fibers do not emit electromagnetic or radio frequency radiation of any consequence and, hence, have negligible impact on the surrounding environment. As an additional advantage, optical fibers are much less sensitive to extraneous radio frequency emissions from surrounding devices and systems. With the advent of optical fiber networks, flexible switching devices are needed to direct light signals between fibers to all-optical domain fiber networks. Commonly assigned U.S. patent application Ser. No. 09/431,430, entitled xe2x80x9cLIQUID CRYSTAL PLANAR NON-BLOCKING Nxc3x97N CROSS-CONNECT,xe2x80x9d and filed on Nov. 1, 1999 on behalf of Thomas M. Leslie et al. discloses a non-blocking Nxc3x97N cross-connect having an array of liquid crystal (LC) switches in a grid of planar optical waveguides within a light optical circuit (LOC). The disclosed LC switches include rectangular trenches or canals formed in a planar optical waveguide that are filled with an LC material. The various LC switches disclosed can function as a waveguide polarization splitter, a transverse electric (TE) switch cross-point, a transverse magnetic (TM) switch cross-point, a waveguide polarization combiner, a filter, variable optical attenuator, or a signal splitter. The disclosed cross-connect system is formed by combining these elements. The disclosed LC switches can be electrically addressed to create an index change that can either match the waveguide conditions or create a total internal reflection condition.
FIG. 1 shows an example of the cross-connect system disclosed in the above U.S. patent application Ser. No. 09/431430 the disclosure of which is incorporated by reference herein. The cross-connect system includes an input port 10, a polarization splitter 20, a TM switch array 30, a TE switch array 40, a polarization combiner 50, and an output port 60. The input port 10 is a linear array of planar waveguides to which an array of fibers can be pigtailed. Similar to the input port 10, the output port 60 is a linear array of planar waveguides to which an array of fibers can be pigtailed. Light from the fibers enters the input port 10 and is passed to the polarization splitter 20.
FIG. 2 shows a more detailed schematic diagram of the cross-connect system shown in FIG. 1 with the exemplary cross-connect being a 4xc3x974 cross-connect. The polarization splitter 20, the switching arrays 30, 40, and polarization combiner 50 are disclosed in the above U.S. patent application Ser. No. 09/431,430 as being formed with the same fundamental LC switch element, which is an LC-filled trench or canal in a planar waveguide, as generally shown in FIGS. 3A and 3B.
The LC switches disclosed in the above U.S. patent application Ser. No. 09/431,430 are polarization dependent and thus, the TE and TM waves are handled separately. Light from the input port 10 enters the polarization splitter 20, which separates the TE and TM waves by reflecting the TE waves to the TE switch array 40 while passing the TM waves to the TM switch array 30. Each switch array 30, 40 has a plurality of LC switch elements 35, 45 in each path 31-34 and 41-44, respectively. A single LC switch element in each path is set to a reflecting state to pass the light onto the polarization combiner 50. The path difference for the TE and TM waves is substantially identical. The polarization combiner 50 allows the TE wave to pass while reflecting the TM wave to recombine. Thus, the beams are recombined and passed to the appropriate path in the output port 60.
FIGS. 3A and 3B show a top view of an LC switch element as disclosed in the above U.S. patent application Ser. No. 09/431,430 in two different states. As shown in FIG. 3A, the LC switch element includes a trench 70 formed at the intersection of a first waveguide 75 and a second waveguide 77. The front sidewall 74 and rear sidewall 76 of trench 70 have an alignment layer disposed thereon and trench 70 is filled with an LC material 72. LC material 72 has a plurality of elongated molecules that align perpendicular to the alignment layers on surfaces 74 and 76 when no electric field is applied through the LC material. Thus, the molecules would be aligned as illustrated in FIG. 3A. As disclosed in the above U.S. Pat. No. 09/431,430, the waveguides 75 and 77 have a refractive index of approximately 1.7 while the ordinary refractive index no of LC material 72 is approximately 1.5 and the extraordinary refractive index ne of LC material 72 is approximately between 1.6 to 1.8.
When a TM wave propagates through first waveguide 75 in the direction corresponding to arrow A in FIG. 3A and when the LC molecules are aligned as shown in FIG. 3A, the TM waves pass through trench 70 and continue to propagate along waveguide 75 in the direction indicated by arrow C. A TE wave, however, propagating in the direction indicated by arrow A along waveguide 75 couples directly into the ordinary ray in the LC material 72, which has an index of no (xcx9c1.5). This index is considerably lower than the effective refractive index of waveguide 75, thus resulting in total internal reflection at front surface 74 of trench 70. Thus, the TE wave is reflected into second waveguide 77 and propagates through that waveguide in the direction indicated by arrow D. Thus, if both TE waves and TM waves are concurrently propagating through waveguide 75 in direction A, the LC switch splits the TE and TM waves from one another while directing the TM wave through first waveguide 75 in the direction indicated by arrow C and transmitting the TE wave through second waveguide 77 in the direction indicated by arrow D. Conversely, if a TM wave is propagating through second waveguide 77 in the direction indicated by arrow B while a TE wave is concurrently propagating through first waveguide 75 in the direction indicated by arrow A, the LC switch functions as a beam combiner by allowing the TM wave to be transmitted through trench 70 and continue to propagate down second waveguide 77 in the direction indicated by arrow D while also redirecting the TE wave through second waveguide 77 in the direction indicated by arrow D.
Referring to FIG. 3B, the LC material 72 in trench 70 is illustrated in an alternate orientation, which would occur when a voltage is applied between two electrodes that are provided on the bottom of the trench and the top of the trench. When the voltage is applied, the molecules of LC material 72 align themselves in parallel with sidewalls 74 and 76 in a vertical orientation.
When the LC molecules are aligned as illustrated in FIG. 3B, a TM wave propagating through first waveguide 75 in the direction indicated by arrow A is reflected from the first surface 74 of trench 70 into second waveguide 77 in the direction indicated by Arrow D. A TE wave propagating through first waveguide 75 in the direction indicated by arrow A would propagate through the LC-filled trench 70 and continue to transmit along first waveguide 75 in the direction indicated by arrow C. The LC switch may thus also function as a polarization beam splitter and beam combiner in the same manner as indicated with respect to FIG. 3A with the exception that the TE and TM waves would be split and directed along different waveguides.
The liquid crystal switch shown in FIGS. 3A and 3B may also be used to selectively direct a light signal having a single polarization state to a different waveguide or allow the signal to continue to propagate through the same waveguide. For example, a TM wave will pass through the LC-filled trench 70 when propagating through first waveguide 75 in the direction A and exit the trench so as to continue to propagate along first waveguide 75 in the direction illustrated by arrow C. By then applying a voltage to the electrodes (not shown), the TM wave may be redirected to instead propagate through second waveguide 77 in the direction indicated by arrow D. A TE wave may similarly be directed along different waveguides by selectively applying a voltage across the LC switch electrodes.
While the LC switch shown in FIGS. 3A and 3B is effective for performing the functions described above, the structure utilizes optical waveguides having relatively high indices of refraction. Materials suitable for use as waveguides and having such high refractive indices (i.e., 1.7) are not available at the same cost that waveguides with relatively low indices of refraction (i.e., 1.5) are available. While it is desirable to replace the higher index waveguides with low index waveguides, one cannot simply replace the higher index material with a lower index material while still utilizing the construction shown in FIGS. 3A and 3B. Specifically, when a low index material is utilized, waves propagating through the waveguide and coupling to the extraordinary wave of LC material 72 would pass through first surface 74 of trench 70 and would be reflected off the inside of the rear surface 76. The angled front surface of the trench would cause further refraction of the light wave and much of the light wave would be lost rather than being properly redirected along second waveguide 77. Accordingly, there exists a need for an LC switch construction that is compatible with low index waveguides.
One aspect of the invention is to provide an optical device comprising first and second optical paths for propagating a light signal, wherein the second optical path intersects the first optical path at a cross-point; and a liquid crystal prism positioned in the first and second optical paths at the cross-point for directing at least a portion of the light signal from the first optical path to the second optical path.
Another aspect of the invention is to provide an optical cross-connect switch comprising a substrate; a plurality of optical waveguides provided on the substrate, wherein each of the waveguides intersects at least another one of the waveguides; and a plurality of liquid crystal prisms each disposed on the substrate at a position where a pair of the waveguides intersect. The liquid crystal prisms selectively redirect a light signal propagating through one of the pair of intersecting waveguides to the other of the pair of intersecting waveguides in response to an electrical signal.
Another aspect of the present invention is to provide an optical device for directing a light signal comprising a substrate; an optical waveguide disposed on the substrate; at least one first electrode disposed between the substrate and the optical waveguide; a liquid crystal prism formed in the optical waveguide; and a top plate connected to the substrate.
Another aspect of the present invention is to provide a dynamically adjustable optical prism comprising a prismatic-shaped structure having an input surface for receiving a light signal, two output surfaces, and two opposed surfaces. The prismatic-shaped structure includes a dynamic material disposed between the surfaces that changes optical states in response to an electrical signal to selectively direct the received light signal to exit through one of the two output surfaces. The dynamically adjustable prism further includes a pair of electrodes each disposed on a different one of the two opposed surfaces for applying the electrical signal to the dynamic material.
Another aspect of the present invention is to provide an optical device for directing a light signal comprising an optical waveguide, and a liquid crystal element formed in the optical waveguide. The liquid crystal element has an ordinary refractive index that is substantially equal to the refractive index of the optical waveguide and has an extraordinary refractive index that is greater than the refractive index of the optical waveguide.
Another aspect of the present invention is to provide a method of directing light comprising the steps of (a) providing a liquid crystal optical element having surfaces configured to function as a prism; (b) directing a light signal at a first one of the surfaces of the LC optical element; and (c) selectively changing the state of the LC optical element to cause the light signal to be directed from different surfaces of the LC element.
Another aspect of the invention is to provide a method of making an optical device comprising the steps of (a) providing a substrate; (b) forming a trench in the substrate having a shape of a prism; (c) dispensing an LC material in the trench; and (d) securing a cover over the LC-filled trench and a portion of the substrate.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention, which, together with their description serve to explain the principals and operation of the invention.